Variable density sound recording method and system



Oct. 31, 1944. I A. c. BLANEY 2,361,451

VARIABLE DENSITY SOUND RECORDING METHOD AND SYSTEM IVENTOR.

ETHUIQ C; AME);

ATTORNEY.

.Oct. 31, 1944. 7 c. BLANEY 2,361,451

'VARiABLE DENSITY SOUND RECORDING METHOD AND SYSTEM Filed Feb. 11, 1943 2 Sheets-Sheet 2 11/0/55 AM L/F/Ee L Beaver 0N AM uF/Ee 0 C 0225417- 75 SHUTTEQ Cams 7071/0/68 INVENTOR Reduction 46 v ATTORNEY.

c rding systems.

Patented Oct. 31, 1944 UNITED s'rAri-z s PATENT orncs VARIABLE DENSITY SOUND RECORDING METHOD AND SYSTEM Arthur C. Blaney, Los Angeles, Calif., assignor;

to Radio'Corporation of America, a corporation A of Delaware -14 Claims.

This invention-relates to sound recording systems, and particularly to variable density sound There are two primary types of sound recording systems used commercially, one known as the Application February 11:1943, Serial No. 475,583 I duced to a predetermined extent. The present invention limits this distortion to a minimum.

variable area type wherein a-trace separating opaque and-transparent areas is produced longitudinally of the film, and the other known as the variable densi type. wherein a series of striations of difieren densities are produced transversely of the film. In variable area recording a light source of constant intensity is usually emnot utilized by the modulations and, in a variable In the recording of both variable area and variable density records, noise reduction is generally introduced which, in a variable area system, is produced by blackening the envelope of the transparent areas of the sound track region density system, by producing a high average density on the final print during no modulations or during the period when'the modulations are of small'amplitude and decreasing this average denployed, the light beam formed thereirom beingvibrated transversely of the direction of film motion. Certain types of variable density systems employ light sources of varying intensity, certain othertypes employ light sources of constant intensity in which the time of exposure varies, while still other types use a constant intensity light source but form the light into a graduated intensity beam by a penumbra shutter, the penumbra being vibrated across a slit to impress the film with light of varying intensity. Although the invention is disclosed as embodied in the latter type of variable density systennit is to be unsity as the modulations increase in amplitude. Thus, in a variable density system the no-signal setting is such as to produce a nearly opaque sound record, the average opaqueness being reduced citthe average transmission being increased as the amplitude of the modulations increase. 7 I It is realized that in the application of noise reduction to either variable area or. variable density systems, a small margin must be provided to accommodate the first cycles of any signal, the

peaks of which would otherwise be lost if no such margin were provided. If this margin is made too derstood that it is also applicable to other forms of modulatingsystems which vary the intesity of or time of exposure of the light impressed on thefilm in accordance with the sound waves being recorded. 7

Since in variable density recording systems the varying amplitudes of the sound waves are translatedin the form of varyinglight intensities, the ilhn emulsion characteristic' becomes an important factor in obtaining the proper relative density gradations on the final film record. It is well large, unnecessary noise is introduced, while it this margin is made too small, excessive peak clipping is the result. Thus, an optimum compromise setting is desirable between these two mown that the H and D curve of a film emulsion limiting conditions which distortion introduced by operating over the curved portion of the film characteristic. With-respect to .the latter, it has beenjfound that it the distorless objectionable than when only the peaks on one side of the wave axis-are distorted. In the past variable density systems .operatedso that the linear range of the film is considerably-exceeded in order to obtain a higher signal level inreproduction. At this high level,

the margin required for low levels is now so high that the maximum in noise reduction is'not oblimit the range of amplitudes recordable to such an extent that this type of recording system would be very ineflicient and not commercially. practicable. Thus, alight variation range is impressed on the film which utilizes the curved portionssof the emulsion characteristic, it being realized that linearity does not now exist over the entire recorded range/but that distortion is intro- 55 the mean or average exposure at tained and distortionis introduced in the form of unsymmetrical peak clipping 'at these. high amplitudes.

The principal object oi'the invention. theremethods of and systems for recording variable density sound recv fore,-is to improve the present ords.

Another object of the invention is to increase the signal level recordable upon a. fllm as a variable densitysound track with minimum distortion and maximum noise reduction. r

A further object of the invention is to control must be related to the.

tion issymmetrical, that is, all wave peaks areequally distorted. that this form of distortion is' have been the higher si nal levels so that the higher density variations will be aprpoximately symmetrically distorted.

A further object of the invention is to provide a variable density recording system in which a higher signal level is reproducible from a given I film record having optimum margin and the minimum amount of unsymmetrical distortion of the sound waves.

Although the novel features which are believed to be characteristic of this invention are pointed out with particularity inthe claims appended herewith, the manner of its organization and the mode of its operation will be better understood by referring to the following description, read in conjunction with the accompanying drawings, in which:

Figs. 1, 2 and 3 are graphs'comparing the distortion of recorded sound waves obtained by the old and new variable densityrecording systems,

Fig. 4 is a detail view of a sine wave of Figs. 1, 2 and 3 illustrating the form of distortion endistortion occurs, but when the level isabove this curve lies halfway between the peak values, thus countered when the rectilinear range of the film emulsion is exceeded,

Pg. 5 is a diagrammatic view of a sound recording system embodying the invention,

F g. 6 is a detail schematic view of the output portion of the noise reduction amplifier of Fig. 5, Fig. 7 is a detail sketch of a magnet c system for operating th shutter of the system of Fig. 5, V.

and

Fig.8 is a graph illustrating the characteristic des red between noise reduction current and shutter travel.

Since it is d fficult to illustrate light variations in a variable density recording system, reference is made to an oscillographic form ofillustration superimposed upon a film characteristic between negative exposure and print transm ssion. Thus,

' in Figs. 1, 2 and 3 curve ABCD represents the relationship between percent negative exposure and percent print transmission for a standard film emulsion, it being noted that this curve 'has a curved toe section AB, a rect linear central sect on BC, and acurved shoulder section CD. It is to be understood that this curve represents the overall relationship between the exposure of a negative film and the transmission of a positive film printed therefrom when processed in accordance with present standards. Varying exposures between points 3 and C will result in providing the maximum in noise reduction and introducing the minimum of distortion. since the peaks are symmetrically distorted. However, such a system is impractical because of excessive peak clipping at the low signallevels.

In Fig. 2 similar curves are shown but for a system in which a margin has been provided to reduce peak clipping, as illustrated by the position of the average exposure curve BGH, the former average exposure curve 'BEF being shown in dotted lines. It will be noted that when a sine wave such as shown in Fig. 1 is now applied to a variable density system so adjusted, the

average exposure is increased so that the optimum average density, from. a noise reduction standpoint, is departed from, as illustrated by the area between the lower peaks of the sine wave and the lower rectilinear transmission point B. In this system, as in Fig. 1, no distortion will occur over the rectilinear section of the emulsion characteristic, but at the higher levels, the upper peaks will be distorted and not the lower peaks until the peak level is increased to bring the exposure below point B, after which symmetrical distortion will occur for all peaks. In this system, therefore, the proper amount of margin has been provided to prevent excessive peak clipping at the low levels, but the maximum in noise reduction is not obtained since'the print will be too light, 1. e., the average transmission will be too high over the medium range of amplitudes at which most recording is done. Thus,

when sufficient margin is provided for such a system, unsymmetrical peak clipping may extend over a considerable range such as that shown in the section above point C, while the print will be too noisy.

an exact corresponding variation in the light transmitted by the final record print. Exposures above point C do not produce corresponding light transmissions and a loss of proportionality exists. Similarly, for the toe section AB.

' However, exposures exceeding the section BC are made on the film to increase the recordable signal range. Noise reduction is introduced by decreasing the mean or average transmission value about which the peaks of the modulations vary. Thus, modulations of low amplitude vary'about a low exposure and low transmission point, this point increasing with increasing amplitude of the signal.

A mean exposure-transmissionv curve is shown by the dot and dash curve BEF which has a re'ctilinear section BE corresponding to rectilinear section BC 'of curve ABCD and a curved section EF" corresponding to section CD. To illustrate the distortion introduced when a sine wave is impressed on the fihn, a sine%wave of increasing amplitude is shown superimposed about the average characteristic curve BEF, it being noted that as long as the peaks are of such amplitudeas not to exceed the rectilinear section of the curve no' Applicants invention, therefore, is directed to a system which provides the optimum margin with respect to an optimum average density of the printfor a maximum recordable range, this type of operation being shown in Fig. 3. In Fig.

3 the general characteristic ABCD'is shown the same a in Figs. 1 and 2, the average exposure curve BGH of Fig. 2 being shown in dotted lines for reference purposes, while the avarage exposure curve in accordance with the invention is shown by clot and dash curve BIJ. Curve BIJ is composed of the lower portion of curve BG and a new curved section extending from I to J. From this illustration it will be observed that no distortion is present over the rectilinear portion of the characteristic and consequently at the lower amplitudes, noise reduction is substantially a maximum as shown in Fig. 1 since the curve BIJ follows margin curve BG only to point I and then substantially centers the average transmission value, while symmetrical distortion is obtained as soon as any distortion'is introduced at the higher levels.

.understod that each 1, 2 and 3 as distorted only on the top, are actu- To illustrate the actual form of the distortion encountered, reefrence is made to Fig. 4, it being of the peaks shown in Figs.

ally distorted starting from a point below the top, which point corresponds to the exposure value where nonrectilinearity begins, such as pointsCandB.

A system for. obtaining the desirable characteristics of Fig. 3 is shown in Fig. 5,where .a typical form of penumbra variable density sound recording system is illustrated. In this system a microphone 5 feeds a pre-amplifier 6, the output of which is divided, one portion being impressed upon anoise reduction amplifier 8 and the other portion upon a galvanometer shown diagrammatically at 9 over conductors ID. The output ofthe noise reduction unit 8 is fed over conductors II to a magnetic unit l2 adapted to move a penumbra shutter M. The optical portion of the system includes a light source IS, a collector lens il, a'mask l8 having a rectangular aperture l9 therein, projecting lenses 22 and 23 on each side of the mirror'of the galvanometer 9, a mask 24 having'a slit 26 therein, and an objective lens 21, the emergent light from which is projected upon the sound track area 29 of a film 30. Such a general type of variable density recording system is shown and claimed in U. S. Patent No. 2,209,053 of July 23, 1940, to G. L. Dimmick.

In brief, this type of system functions as follows:'-

The light beam passing the aperture i9 is formed into a penumbra by the shutter M, which penumbra is vibrated normal to the slit 26 by the mirror of the galvanometer 9, varying in-;

tensities of light being projected to the film 30 in accordance with th instantaneous ampli tudes of the sound waves impressed upon the microphone 5 and the .average value of .these waves by varying positions of shutter Hi. The position of the penumbra with respect to the slit 26' is thus varied by both the-galvanometer 9 but also by the shutter l4 which is actuated by the noise reduction currents from unit 8. In order to provide an average exposure characteristic similar to curve BIJ in Fig. 3, the relationship between the average value of the signal'currents or the rectified output current to the shutter coils and the shutter movement or travel should be as shown in Fig. 8, the initial current setting being on the rectilinear section of the curve of Fig. 8 to correspond to point B in Fig. 3.

the non-proportionality desired between a cur-' rent corresponding-to the average value of the signal current and shutter travel is illustrated. This magnetic unitcan then besubstituted for the unit shown at, l2 in Fig. 5, and the noise reduction amplifier 8 may be any standard type. In Fig. 7 the unit is shown asfhaving two magnetic paths formed by electromagnetic sections 44 and 55, the conductors H from the noise reduction amplifier 8 being connected in series around both sections. An armature 46 is positioned in the air gaps of the sections 44 and 45 and is attached to the shutter M. In this arrangement the output current is adjusted so that saturation of the magnetic circuit begins at point I on the exposure-transmission characteristic shown in Fig. 3, the B and H characteristic of the magnetic structure having a shoulder portion of a curvature such as shown in Fig. 8 which is utilized to produce the section IJ in Fig. 3.

It is realized that other methods of and means for obtaining the proper non-rectilinear char-v acteristic over the upper amplitude ranges may suggest itself to those skilled in the art, the above-described means shown in Fig. 6 having operated satisfactorily in a system such as shown in Fig. 5. When the characteristic of the magnetic system of the shutter is relied upon, the output current of the noise reduction amplifier may be directly proportional ,to the average value of its input signal, although both modifications may be'used simultaneously if desired.

I claim as my invention:

1. A sound recording system comprising means for producing a light beam, 'a film, means for modulating the exposure of said film to saidbeam in accordance with the instantaneous valuesiof sound waves to be recorded, and means for modulating' the average exposure of said film to said This current-shutter travel relationship-may be obtained either electrically in the noise reduction amplifier or magnetically in the shutter driving element.

' In Fig. 6 the last two DC stages of the, noise reduction amplifier 8 are shown bytubes 35 and 36, the plate of tube. 35 being coupled-through a battery 38 or other suitable supply to the grid of tube, an isolating condenser 39 being provided between the plate of tube 35 and ground. To obtain the proper non-proportional relationship between the actual average value of the signal currents and'the shutter travel the 7 output circuit of'tub'e- 38 is loaded bya limiting resistor 4|, located between the plate of this tube and the plus-B supply. The value of resistor 4| .is chosen with respect to the value of the 13 potential so that this' resistor controls the amount or output voltage available with changes light beam directly in accordance with the average peak variations of said sound waves over the lower range of amplitudes of said sound waves toprovide an average film transmission directly proportional to exposure, saidmeans varying the average light intensity ina predetermined nonproportionality over the upper amplitude range of said sound waves, said upper amplitude range including a range of amplitudes which would normally provide a' direct proportionality bet'wen film transmission and'exposure.

2. A sound recording system in accordance with claim '1, in whichthe variation. inaverage exposure substantially follows thegeneral curvature of the exposure and transmission characteristic beginning below the midpoint on the rectilinear portion of saidcharacteristia 3. A variable density} sound. recording] system comprising a, light source, means for forming light from said source: into I a. beam, means ..for graduating the intensity-Uot said, beanil over' a portion thereof, means for varying the "position of saidselected portion of said graduated beam in accordance with the instantaneous values of sound waves to be recorded, a film on which said light beam is impressed, and means for varying the average position of the selected portion of in the plate impedance of tube 36. In this m'anher as the average value of the signal currentincreases, the output current to the shutter is increased non-proportionally to move the shutter. as shown in Fig. 8 which produces the curve IBII'ofFlgB. i a

I In Fig. ,7 .the magnetic portion of'a penumbra shutter mechanism which in itself will-produce said beam in accordance with a predetermined departure from the variation in the average value of said sound waves, said average light intensity varying rectilinearly over the lower amplitude range ofsaid sound waves and non-proportion ally over the upper amplitude range of said sound wav 5, said non-proportionality. beginning below the dpoint on the. rectilinear portion of the average transmission-exposure characteristic, and continuingbeyond a point corresponding to the midpoint on said characteristic. 4

4. A vsound recording system in accordance with claim 3, in which said non-proportional variation of said average position of said selected portion of said light beam begins at a point substantially midway between the central point and one end of the rectilinear portion of the characteristic between negative exposure and print transmission of a film emulsion.

5. The method of recording a variable density sound record comprising modulating a light beam in accordance with the instantaneous amplitude values of sound waves being recorded, modulating said beam in accordance with the average amplitude values of said waves, said second menthe said characteristic over the upper amplitude range of said sound waves.

,9. A sound recording system in accordance with claim 8 in which said last mentioned means includes electronic means for varying a current corresponding to the average value of said sound waves in substantial accordance with said characteristic.

-10. A sound recording system in accordance with claim 8 in which said last mentioned means includes magnetic means ror varying th a ra intensity of said beam, the relationship between the current input to said magnetic means and the variational the average intensity of said beam produced thereby being in substantial accordance V with said characteristic.

tioned modulation varying the average modulation of said beam rectilinearly over the lower 7 range of amplitudes of'said sound waves and non-rectilinearly over the upper range of ampli-,

tude levels of said sound waves, and impressing a film to a light beam in accordance with the instantaneous amplitude values or sound waves,

said exposure varying rectilinearly over a portion of the amplitude range'oi said sound waves and non-rectilinearly over the remaining range of amplitudes, and varying the exposure to said light beam in accordance with the average amplitude values of said sound waves, said last-mentione'd variations being non-rectilinear from a point below the central point on the exposuretransmission characteristic.

8. A sound recording system comprising means for forming light into a. beam, a film, means for exposing said film to said beam, and means for modulating said beam in accordance with sound waves, said last mentioned means including means for rectilinearly varying the average exsound waves.

11. A variable density sound recording system comprising means for forming light into a beam,

:3. film, and means for simultaneously varying the exposure of said film to said beam in accordance with the instantaneous values and the average vaiues'of soundwaves being recorded, said average value exposure varying means including a rectifier of currents corresponding to said sound waves and a direct current amplifier for the rectified currents, said amplifier having its plate circuit loaded to limit the plate current therefrom over the higher amplitude range of said 12. A variable density sound recording system comprisin a light source, means for forming light from said source into a beam, means for exposing a film to-said light beam, means for modulating the exposure of said film by said light beam in accordance with the instantaneous values of sound waves, and means for controlling the average exposure of said film by said light L beam to produce on aprint from said film having a-transparency varying between thetransparency corresponding to one end of the rectilinear portion of the transmission-exposure characteristic of said films and a point below the central point on the rectilinear portion of said characteristic.

posure with sound wave amplitude over the lower amplitude range of said sound waves and varying said avera e exposure non-rectilinearly over the upper amplitude range of said sound waves, nonrectilinearity in the average-intensity beginning below the midpoint on the rectilinear region voi the ne ative exoos re orlnt transmission characteristic of saidflim and thereafter varying in substantial accordancewith the upper portion 0! w 13. A variable density sound recording system in accordance with claim 12 in which said last mentioned point is substantially midway between.

said central point on said characteristic and said one end of the rectilinear portion ofsaid characteristic. a

14. A variable density sound recording system iii-accordance with claim 12 in which said average exposure above said point on said rectilinear portion of said characteristic varies in substantially the same relationship as said on-exposure characteristic and extends above a point nding to said central pointon the rectiear portion of said characteristic.

7 ARTHUR C. BLANEY. 

